Path finder antenna

ABSTRACT

An antenna for cellular telephone communications systems, particularly intended for base stations (RPF) of DECT standards, which is able to search for the best path to the user, is formed as a multimode, adaptive, dual antenna, apt to take up both a narrow lobe configuration, with variable orientation on a horizontal plane, and an omnidirectional configuration on a horizontal half-plane, the two antennas composing said dual antenna being similar, integrated onto the same dielectric substrate, and working simultaneously with two different roles (traffic support; search for optimal orientation), said roles being exchanged at every receipt-transmission cycle.

It is well-known that the so called CTM (Cordless Terminal Mobility) hasbeen recently more and more developed in the telephone field, due to theextension to a whole town of the function of DECT (Digital EnhancedCordless Telecommunication) apparatuses—namely of cordless telephoneapparatuses which function in confined areas—achieved by means of adirect connection to central apparatuses of the fixed network.

It is also well-known that the DECT standard, which was created forsmall areas and for indoor use, reaches its limits when employed inlarge areas and outdoors.

The low power of the radio signal is one of the most obviouslimitations. The weakness of such a signal together with the use of arelatively high frequency, meaning a high likelihood of reflections andinterferences, makes communication precarious as soon as the distancebetween the base station RFP (Radio Fixed Part) and the user's Mobile PP(Portable Part) increases.

The interference to be taken into account in the DECT standard is notcaused by signals with the same frequency, coming from different basestations, as in the GSM standard.

This is because the choice of the transmission frequencies is madeautomatically and dynamically by the RFP-PP system, by sensing thefrequencies used by adjacent systems, using a different frequency, so asto avoid at the beginning this kind of interference. (The synchronismbetween base stations, typical of the DECT standard, ensures the correctsensing of the frequencies already used).

Destructive interference in the DECT standards is determined by signalscoming from the same source, with the same amplitude, but reaching theantenna with opposite phase: this is caused by the existence of multiplesignal paths from the transmitter to the receiver, characterized byreflections in different directions, with different path lengths, butwith a similar attenuation.

Communication by reflection is particularly relevant in DECT standards,because of the high frequency used (the wavelength is comparable withthe size of objects present in the town environment) and of thecomparatively low location (4-6 meters) of base stations from theground, that does not allow the illumination of users from above.

The presence of common obstacles, like buildings, trees and vehicles,leads to very diversified multiple paths, with a high likelihood ofphase shifts and delays. Small changes in the PP position (of the orderof a few wavelengths) are sufficient to come out of the opposite phasecondition, and thus of interference, or to get into it. This is thereason why is it not uncommon to see DECT users moving and turning thePP in an attempt to obtain the best signal (i.e. with the highest powerand free from interferences).

The solutions adopted in existing installations consist in reducing thearea covered by a single base station RFP, to enhance the average levelof the available radio signal, and in making use of the so called“antenna diversity” to obviate interferences.

However, to reduce the area covered by a single RFP means increasing thenumber of the RFP required by the system, with evidently higherinstallation and maintenance costs.

The antenna diversity is obtained by making use of two antennaspositioned at least two wavelengths apart, if their polarization is thesame, or even less if their polarization is different. Such diversityshould ensure that, in case the signal received by one antenna isattenuated by reflected, interfering signals, the other antenna receivesa signal that can be utilized, because of different geometric conditionsleading to different interference conditions.

The use of polarization diversity (vertical polarization and horizontalpolarization) to obtain antenna diversity cannot be avoided if bothantennas have to be contained within the same case as the electronicpart, and if the overall dimensions of the whole base station have tofulfil market requirements (maximum dimension below 300 mm and volumebelow 5 liters).

On the other hand, the polarization of the wave impinging on the RFPantennas cannot be predicted (because it is determined by the PPorientation and the effects of consecutive reflections). Therefore, alinearly polarized antenna is not always able to receive the maximum ofthe available field: from this point of view, a circular polarizationantenna is certainly more effective.

The best use of the antenna diversity, however implemented, is now theobject of much research, aiming at the development of algorithms, mainlybased on statistical models of the environment in which the RFP's willoperate.

Of course, following this route, any possible solution which might bereached will suffer from a close dependence on the environment and beaffected by the precariousness of the statistical data.

The present invention follows a totally different route—and fullyoriginal, at least in the DECT technology—which is based on the idea ofletting the antenna of base stations search for the best communication,which search is nowadays carried out by the user in difficulty.

To realize such an idea, it has been observed that the search for thebest communication is normally carried out by the user by altering thegeometric configuration of the RFP-environment-PP system, by moving andturning the PP, and by making use of the information resulting fromthese changes. By perfect analogy, the RFP must change the configurationand the orientation of its antenna to search for the optimal geometricconfiguration of the RFP-environment-PP system, through real changes andthe use of the information obtained as a consequence of the changes,rather than through statistical considerations.

In other words, it is not the user who must search for the antenna, butrather, it is up to the antenna to find and follow the user, and thiscan be obtained by seeking the antenna configuration which maximizes thesignal and minimizes the effects of a possible interference.

The notion that the directional characteristic of the fixed base stationin a mobile radio telephone system can be matched to the currentlocation of a mobile user has been already disclosed in WO-A-o6/29836,describing a system which manages a multiplicity of elementary, narrowlobe antennas, each with different orientation, choosing time to timethe elementary antenna among said multiplicity of antennas which is bestorientated to be matched to the mobile station of the user.

The present invention reaches the same aim, but advantageously using,instead of a multiplicity of elementary antennas, a sole antennaconsisting of a plurality of “patches”, in which, varying the phasebetween “patch” and “patch”, the orientation and the width of theantenna lobe are changed, to obtain the wished best connection with themobile user. This antenna may be in fact omnidirectional in a horizontalhalf plane or be with narrow lobe and orientated in a very good way,with a reduced size, compatible with the DECT standards.

More precisely the antenna according to the present invention, intendedfor cellular telephone communication systems and in particular for basestations (RFP) of DECT standards is characterized in that it is formedas a multimode, adaptive, dual antenna, apt to take up both a narrowlobe configuration, with variable orientation on an horizontal plane(azimuthal plane), and an omnidirectional configuration on an horizontalhalf-plane, the two antennas composing said dual antenna being similar,integrated on the same dielectric substrate, and working simultaneouslywith two different roles (traffic support; search for optimalorientation), said roles being exchanged at every receipt-transmissioncycle.

Advantageously, both said antennas forming the dual antenna consist of aset of “patches”, phase shifters being interposed between them and beingproduced by identical technology on the same substrate.

The two component antennas may be provided on the same substrate eitherwith discrete sets of patches and phase shifters, or with discrete setsof phase shifters and with common patches, used with differentpolarizations.

Circular polarizations can be used for said patches, a clockwisepolarization for one antenna and a counterclockwise polarization for theother, or else a vertical polarization for one antenna and an horizontalpolarization for the other.

The invention also relates to an antenna for cellular telephonecommunication systems which is able to search for the best path to theuser matching the directional characteristic of a fixed base station tothe current location of a mobile user, characterised in that is formedas a multimode adaptive single antenna consisting of a set of patches,phase shifters being interposed between them and being produced byidentical technology on the same dielectric substrate, said antennabeing apt to take up both a narrow lobe configuration, with variableorientation on an horizontal plane, and an omnidirectional configurationon an horizontal half-plane, so as to be able, in successive periods, tosearch for optimal orientation and to support the traffic.

The invention is now described in greater detail, reference being madeto some preferred implementations thereof, illustrated on theaccompanying drawings, wherein:

FIG. 1 shows a first, possible implementation of the antenna accordingto the invention;

FIG. 2 shows a second, possible implementation of the antenna accordingto the invention;

FIGS. 3 and 4 are irradiation diagrams of the antenna, taken on thehorizontal plane in the narrow lobe configuration (with circularpolarization), which show the removal of the interference and,respectively, the variable orientation; and

FIG. 5 is an irradiation diagram of the antenna, taken on the horizontalplane in the omnidirectional configuration (with circular polarization).

The antenna according to the invention is a multimode adaptive dualantenna, able to take up both a narrow lobe configuration, with variableorientation on the horizontal plane, and an omnidirectionalconfiguration on the horizontal half-plane, which consists of twosimilar component antennas, integrated on the same dielectric substrateand working alternatively with exchanged roles for very short periods,so as to be able to simultaneously provide both communication and searchfor optimal orientation (namely, the best path). A circular polarisationof the antenna is preferred.

More exactly, in a 10 ms period, the first antenna handles the traffictransmitting during the first 5 ms and receiving during the following 5ms, while the second antenna is switched off during the 5 ms of thetransmission, finding and recording the optimal orientation for eachuser in the next 5 ms of reception.

In the following 10 ms, the roles of the two antennas are exchanged and,while the first one searches for optimal orientation, the second onemakes use of the information just obtained about optimal orientation totransmit and receive.

In this way, information on orientations is updated every 10 ms, withoutbeing affected by possible differences between the two antennas. Itshould be taken into account that a user, moving at a speed of 10 km/h,covers a distance of 2.8 cm in 10 ms.

FIG. 1 shows a possible first implementation of the antenna according tothe present invention, with which a narrow lobe on the horizontal planeis achieved. In this implementation, the antenna extends horizontalwiseon the same dielectric substrate using two discrete sets of patches andphase shifters, one set for each of the two component antennas.

Each set comprises five patches 1, connected in series, and four phaseshifters 2, inserted between said patches and controlled so as to allgive the same phase shift. The use of five patches is a good compromisebetween the performances obtained (a sufficiently narrow lobe) and thecomplexity and cost of the antenna.

Square patches are shown in FIG. 1, but also circular patches could beused with equally satisfactory results.

The phase shifters of both sets are controlled by two analogue inputs 3and force a phase shift among patches that is constant over the range ofuseful frequency.

By introducing additional phase shifts between two consecutive patches,up to =90°, it is possible to rotate the orientation of the main lobe upto ±70°.

The phase shifters should be able to shift their phase by an extentwhich continuously varies from 0° to 180°.

The total phase shift, which is given by summing the phase shiftintroduced by the phase shifter to the one introduced by theinterconnecting strip-lines 4 should vary from 360°−90° to 360°+90°: byshifting the phase between two consecutive patches up to +90°, thefourth Quadrant is covered; by shifting the phase between twoconsecutive patches up to −90°, the first Quadrant is covered; while, byintroducing phase shifts of +90° between the first patch and the secondone and between the second patch and the third one, and by introducingphase shifts of −90° between the third patch and the fourth one andbetween the fourth patch and the fifth one, the omnidirectional antennais achieved.

FIG. 2 shows a second, more complex implementation of the dual antennaaccording the invention, wherein the two component antennas are providedon the same substrate with two discrete sets of phase shifters andcommon patches.

In this case, ten common patches 11 are provided, reciprocally connectedas shown in FIG. 2, and two sets of four phase shifters 12 are insertedbetween said patches and are controlled by analogue inputs 13.

In an antenna thus conceived, all the patches are activated in circularclockwise polarization, so as to provide one component antenna, and incircular counterclockwise polarization, so as to provide the othercomponent antenna.

Alternatively, the patches could be activated in vertical polarizationfor one antenna, and in horizontal polarization for the other.

The antenna shown in FIG. 2, which is more complex and thus moredifficult to implement, but not much more expensive to be produced,allows to reduce the lobe width also on the vertical plan (elevationplane), and thus to increase by 3 dB the maximum gain over the antennashown in FIG. 1.

Therefore, the antenna shown in FIG. 2 is the best implementation ofthis invention, because it maximizes one of the main features thereof.The irradiation diagrams shown in FIGS. 3, 4 and 5 result from asimulation of the antenna shown in FIG. 2.

It is now possible to place in evidence the results which the variousfeatures of the antenna according to the invention allows to achieve.

As to the possibility for the antenna to take up a narrow lobeconfiguration, with orientation variable on the horizontal plane, itshould be noted that:

the narrower the lobe, the higher the antenna gain;

the narrower the lobe, the smaller the likelihood that two signals,originated from the same source, but undergoing different reflectionsand thus reaching the antenna with different phases, are received withthe same intensity.

If one of the signals reaches the antenna close to the direction ofmaximum gain, while the other comes from a direction corresponding to anull of the antenna gain, it is then obvious that the annulment ofinterference is achieved, as clearly shown in FIG. 3.

With a range of orientation change between −70° and +70° and with a lobewidth of 40° at −3 dB from maximum gain, it is possible to cover all the180° of a half-lane. FIG. 4 shows some of the possible positions of themain lobe.

Of course, with an orientation change ranging between −70° and +70°, itwill not be possible to have maximum gain in all directions, but someimplementation problems are thus eased. For instance:

the reduction to a suitable size of secondary lobes is easier;

the attenuation in the phase shifters, necessary to rotate the antenna,is less, and their implementation is easier and cheaper;

furthermore, although the antenna maximum gain can be oriented onlybetween −70° and +70°, the existence of two nulls delimiting the mainlobe allows to position one null in any direction between −90° and +90°;and

it is always possible to position a null in the direction of theinterfering signal.

As to the possibility for the antenna to take up an omnidirectionalconfiguration on the horizontal half-plane, it should be noted that,although a narrow and precisely oriented lobe is surely the best way tohandle an ongoing link, the initial stages of “dummy bearer”transmission (to signal the presence and availability of the basestation) and call setup require to respect the concept ofomnidirectionality on which the DECT standard is based. Thus, theavailability of an omnidirectional antenna configuration is essential.FIG. 5 shows the radiation pattern of the “Path Finder Antenna”according to the invention in omnidirectional configuration.

Of course, an important advantage of the antenna according to theinvention is the possibility to simultaneously take up both the abovecited configurations, by integrating two component antennas on the samedielectric substrate, said antennas being able to alternate in theirfunction.

Finally, as to the use of circular polarization, it should be taken intoaccount that it is common for the user to experience a change in thefield strength with different orientations of his or her own portabletelephone.

Circular polarization of the base station antenna is intended toeliminate these changes, making the turning of the mobile PP unnecessaryand useless (since the quality of the reception is constant with anyorientation of the Mobile PP) according to the philosophy of the presentinvention.

The reduction of antenna gain caused by the increased power needed forfeeding the antenna in circular polarization should, on the other hand,be compensated (according to available literature) by the statisticalgain due to its inherent capability to receive and transmit, with themost appropriate polarization, the radio signal from a PP having linearpolarization and undefined orientation.

It is also appropriate to check, at least as a first approximation, theavailability of a timing which is sufficient for the use of the antennaaccording to the present invention.

With a simple optimum criterion, like the individuation of theorientation allowing to receive the highest level of radio signal, thefollowing assumptions for the duration of various operations can bemade:

1 μs: time for antenna orientation;

3 μs: time for the measurement of the radio signal level;

2 μs: time for data estimate and recording;

Total time for a measurement: 5 μs.

In such conditions, it is possible to perform around 70 measurements foreach user (a time slot cleaned from guard and synchronization bits is388 bits, corresponding to 336 μs) and thus, even with the most naivemeasurement strategy, a precision of ±1° can be achieved.

A more sophisticated optimum criterion, taking into account the extentof the received data jitter (the jitter for data with a relatively highradio signal can be caused only by multiple interfering paths), as wellas the level of radio signal, requires longer measurement times and moreelaborated measurement strategies.

The antenna according to the invention allows considerable and evidentadvantages, particularly:

It enables to increase the antenna gain in all directions by at least 6dB, in respect to the current standard solutions. In fact, as theantenna lobe on the horizontal plane has been concentrated from 180° to40°, the corresponding increase of antenna gain is the following:

G.I.=10 Log (180/40)=6.5 dB

Antenna gain means, of course, maximum antenna gain, but by changing thelobe orientation this maximum is available, practically, in alldirections.

As the base stations are located approximately at the same height of thePP's and in the same environment, also the conditions of environmentalnoise will be similar, whereby the uplink and downlink paths will besubstantially similar. Therefore the optimum orientation for the uplinkis the optimum orientation also for the downlink.

On the basis of the above, the goal of doubling the radius of thecovered area of each base station, i.e. reducing to ¼ the number of thenecessary RFP in a DECT standard, can be considered reached, at leastfrom a theoretical point of view.

The significant reduction of the interferences, and the independence ofthe communication from the PP orientation (due to the circularpolarization) and from the user's mobility (due to the continuousoptimization of antenna orientation), provided an improvement in thequality of the service which is easy to perceive, even if difficult toquantify in terms of further increase in the radius of the coveragearea.

The limited size of the antenna according to the invention (about230×180 mm) allows to produce base stations also of limited size (muchsmaller than the conventional ones) with a high reduction of the visualeffect and an evident aesthetical improvement.

The high gain and directivity of the path finder antenna according tothe invention should also make it interesting for the RLL (Radio LocalLoop) installations, as least as an inexpensive solution.

A solution more aiming at the RLL installations (substantially involvingfixed users, whereby the search for optimum orientation can betemporarily limited to the “start-up” and to a periodic “refresh”) couldconsist of a multimode adaptive single antenna, apt to search foroptimum orientation and to support the traffic in successive periods,and thus engaged in the search for optimum orientation (which remainsunvaried) only for a very short period, and always substantiallyavailable to support the traffic.

An antenna thus conceived would still be an antenna according to theinvention, but not a dual antenna such as the one previously describedand illustrated. An implementation thereof could for example berepresented by a diagram corresponding to the upper half or to the lowerhalf of FIG. 1.

A single antenna of this type could also be conveniently used, inreception, by DECT Repeaters (WRS=Wireless Relay Station).

Furthermore, it can be seen that the concepts and solutions on the basisof which the present invention has been developed can be applied to anykind of time-sharing radio communication using frequencies close to 2GHz, the only warning being that the information of optimum orientationis available at a given instant and is used at a subsequent moment. Itis important that the user's speed of movement be limited (like in theCTM case), for the information of optimum orientation to be meaningful.

What is claimed is:
 1. An antenna for cellular telephone communicationsystems, particularly intended for base stations (RFP) of DECTstandards, which is able to search for the best path to the user,matching the directional characteristic of a fixed base station to thecurrent location of a mobile user, characterized in that it is formed asa multimode, adaptive, dual antenna, apt to take up both a narrow lobeconfiguration, with variable orientation on an horizontal plane, and anomnidirectional configuration on an horizontal half-plane, the twoantennas composing said dual antenna being similar, integrated onto thesame dielectric substrate, and working simultaneously with two differentroles (traffic support, search for optimal orientation), said rolesbeing exchanged at every receipt-transmission cycle.
 2. An antenna as inclaim 1, wherein both said component antennas consist of a set ofpatches, phase shifters being interposed between them and being producedby identical technology on the same substrate.
 3. An antenna as in claim1, wherein said two component antennas are provided on the samesubstrate with discrete sets of patches and phase shifters.
 4. Anantenna as in claim 1, wherein five patches and four phase shifters areprovided for each antenna.
 5. An antenna as in claim 1, wherein said twocomponent antenna are provided on the same substrate with discrete setsof phase shifters and with common patches, used with differentpolarizations.
 6. An antenna as in claim 5, wherein ten common patchesand two discrete sets of four phase shifters are provided.
 7. An antennaas in claim 1, wherein circular polarizations are used for said patches,a clockwise polarization for one antenna and a counterclockwisepolarization for the other.
 8. An antenna as in claim 1, wherein avertical polarization is used for the patches of one antenna and ahorizontal polarization is used for the patches of the other.
 9. Anantenna for cellular telephone communication systems, which is able tosearch for the best path to the user, matching the directionalcharacteristic of a fixed base station to the current location of amobile user, characterized in that is formed as a multimode adaptivesingle antenna consisting of a set of patches, phase shifters beinginterposed between them and being produced by identical technology onthe same dielectric substrate, said antenna being apt to take up both anarrow lobe configuration, with variable orientation on an horizontalplane, and an omnidirectional configuration on an horizontal half-plane,so as to be able, in successive periods, to search for optimalorientation and to support the traffic.